Ion-selective electrodes are electrochemical sensors which respond to the concentration of specific ionic species in sample solution. The principle for measurement is based on the selective permeability of a membrane for specific ionic species in the sample solution. The response of these sensors depends upon the magnitude of the potential developed across an ion-selective membrane which separates two solutions. i.e., an internal reference solution and the sample solution. The membrane acts as an ion-exchanger which selectively takes up the specific ions leaving counterions behind. This separates the charge at the membrane surface inducing a phase boundary potential.
The potential of the ion-selective electrode is measured against an external reference electrode which Provides a fixed potential that is in dependent of any ionic species in the sample solution. An internal reference electrode, which is an integral part of the ion-selective electrode, provides a fixed potential that is determined by the known concentration of the specific ionic species in the internal reference solution. The measured potential is related to the concentration of the specific ionic species in the sample solution by the well known Nernst equation, EQU E=Constant-RT/F ln a.sub.F -
where the constant term includes the standard or zero potential of the ion-selective electrode. i.e., the potential of the electrode when the effective concentration is unity; the reference electrode potential; and the junction potential, a.sub.F - is the effective concentration of the specific ions in the sample solution, R is the gas constant(8.316 J/mol-deg), T is the absolute temperature, and a.sub.F -is the Faraday constant (96.491 coulombs). Many authors have described ion-selective electrodes and their use for numerous ionic species [Freiser edited, Ion Selective Electrodes in Analytical Chemistry, Volumes 1 and 2, Plenum Press (1978 and 1980). Koryta, Anal. Chim. Acta Vol. 183, 1-46 (1986), and Arnold and Solsky, Anal. Chem. Vol. 58, 84R-101R (1986)].
The importance of determining fluoride ion concentrations in various Products as well as in natural and biological materials is well known [Moody and Thomas, Ion-Selective Electrodes in Analytical Chemistry, Freiser ed., Plenum Press, Volume 1, 339-433, 1978]. However, conventional analytical techniques such as gravimetric or volumetric methods are tedious and labor intensive. Fluoride ion-selective electrodes have been widely used for both scientific and industrial applications because of their ease of use, reliability, exceptional selectivity and sensitivity. Performance of the ion-selective membranes is affected by various factors such as as the aqueous solubility of the active component, ionic conductivity, seal strength at the contact point with an internal reference solution thermal expansion properties as well as the process by which the membrane is prepared. It is well known in the art that pure crystalline rare earth fluorides have high electrical conductance resulting from mobile fluoride ions within defects in the crystal lattice [Sher et al, Phys. Rev. Vol. 144, 593-604 (1966)]. Many of these crystalline fluorides are also water insoluble which make them ideally suitable for use as membranes in high sensitivity fluoride ion-selective electrodes.
The first fluoride ion-selective electrode employing a non-porous membrane of substantially insoluble crystalline fluoride, i.e., the trifluorides of bismuth, scandium, yttrium and the lanthanide series of rare earth metals, and lead fluoride has been described by Frant and Ross in Science, Vol. 154, 1553-1555 (1966) and by Frant in U.S. Pat. No. 3,431,182 issued on Mar. 4, 1969. The most successful membrane disclosed by Frant is a single-crystal of pure lanthanum trifluoride which has a tysonite structure. The fluoride electrode is prepared by sealing a disk-shaped section of the lanthanum trifluoride crystal into a rigid, polyvinyl chloride tube, which is filled with a solution of sodium fluoride and sodium chloride. Electrical contact is made with a silver/silver chloride reference electrode, which is inserted into the internal reference solution. The single-crystal electrodes have a detection limit of 10.sup.-6 M fluoride ions, rapid response time of less than 30 seconds and stable potential measurement capability of 1 mV. These electrodes are useful for various applications but not as single-use sensors for clinical applications because of the high cost involved in membrane preparation. Single crystal membranes require expensive, highly pure optical grade raw materials and time consuming processes. Other problems include tight sealing requirement of the fragile membrane to a variety of sensing electrode configurations and poor thermal expansion properties.
The construction of solid-state fluoride electrodes, in which a silver/silver fluoride contact replaces the internal solution contact, has been reported by Fjeldly and Nagy, J. Electrochem. Soc. 127, 1299-1303 (1980), and Bixler and Solomon, Anal. Chem. 56, 3004-3005 (1984).
There is a need for fluoride electrodes which are of sufficiently low cost to permit them to be used as disposable sensors for clinical applications.
Numerous attempts have been made to prepare such low cost fluoride ion-selective membranes, however, none were successful in overcoming the known disadvantages while maintaining or improving the sensitivity of the single crystal electrodes.
Pungor, et al in U.S. Pat. No. 3,446,726 issued on May 27, 1969 discloses the fabrication of low cost, heterogeneous ion-selective membranes comprising silicone rubber containing small particles of ionic conducting inorganic precipitates. The process was successful for producing silver iodide and barium sulfate membranes. The membranes thus prepared were easily manufacturable, mechanically rigid with great elasticity, resistant to thermodilatation, and highly conductive. However, similar membranes containing polycrystalline fluorides were not disclosed.
An attempt to produce silicone rubber membranes containing powdered lanthanum trifluoride, thorium tetrafluoride, or calcium difluoride has been described by McDonald and Toth in Anal. Chim. Acta Vol. 41, 99-106 (1968). The electrodes thus prepared from lanthanum trifluoride precipitates showed ion selectivity over a narrow range of fluoride ion concentrations from 10.sup.-4 to 10.sup.-2. The most stable fluoride electrode was one prepared with calcium difluoride precipitates. However, sensitivity was poor with a detection limit of 10.sup.-4 M. In addition, resistance of these electrodes was very high which requires high impedance electrometers for potential measurement.
Yet another attempt to prepare heterogeneous fluoride electrodes has been disclosed by Radhakrishna et al in U.S. Pat. No. 3,787,309 issued on Jan. 22, 1974. Electrodes in this invention are constructed using a sintered membrane containing insoluble inorganic salts such as lanthanum fluoride incorporated into a polyalkene resin. Sensitivity improved slightly to 5.times.10.sup.-5 M but is not as good as the single crystal lanthanum fluoride electrode.
Still another attempt to produce low cost fluoride electrodes involves a vapor deposition technique which coats a thin layer of polycrystalline lanthanum trifluoride onto a metal or metal-coated base material [Fait et al, GB 2,163,457A issued on Feb. 26, 1982]. Electrodes of this type can be produced by mass fabrication techniques and have good sensitivity down to 1.times.10.sup.-5 M fluoride ions. However, the high resistance of polycrystalline fluoride may require high impedance electrometers, and the potential measurement would be inherently less reliable since it lacks a stable internal reference electrode.
Other attempts directed at improving the performance of the membranes include: use of a spherical shape membrane comprising a single cyrstal fluoride of a lanthanide mixture [Pungor et al, U.S. Pat. No. 4,021,325 issued on May 3, 1977]; use of lanthanum trifluoride doped with europium difluoride [Frant and Ross, Science Vol. 154, 1553-1555 (1966) and Bausova et al, J. Anal. Chem. U.S.S.R. Vol. 28, 2042-2044 (1973)]; use of ceramic membranes comprising a mixture of lanthanum fluoride, europium difluoride and calcium difluoride [Hirata and Ayuzawa, Chem. Lett. 1451-1452 (1974)]; and use of ceramic membranes of sintered cerium fluoride doped with rare earth metals such as europium, samarium or ytterbium [J 77-013,956 issued on Apr. 18, 1977].
Use of a spherical shape membrane eliminated the thermodilatation problem of the single cyrstal electrodes but the cost disadvantage was still high. Ceramic membranes showed high sensitivity and selectivity, comparable to the single-crystal lanthanum fluoride electrode but the processes required high temperature and pressure not easily attainable in manufacturing. The process for preparing these sintered membranes was further complicated by the need to obtain a non-porous structure. In addition. Hirata's process [Chem. Lett. 1451-1452 (1974)] required an atmosphere of corrosive hydrogen fluoride. Although europium doping of lanthanide metal fluorides are known to enhance ionic conductance, the real advantage in such fluoride electrodes is questionable. Doping is accomplished by adding a small amount of europium difluoride, 0.1 to 0.15 mole % which presumably helps to form a nonstoichiometric crystal with holes for fluoride ions to move around. However, europium difluoride is very unstable thus at least in the membrane surface which is exposed to dissolved oxygen in solution, the europium is expected to be in a +3 oxidation state. Then, since the solubility product of europium trifluoride is about ten times larger than that of lanthanum trifluoride, addition of large quantities of europium Would decrease sensitivity of the membrane. [Lingane, Anal. Chem., Vol. 40, 935-939 (1968). Moody and Thomas, Ion-Selective Electrodes, 69-70, published by Merrow, England, 1971].
Supersonic conducting ternary compounds have been known in the art for some time [Nagel and O'Keefe, Fast Ion Transport in Solids, 165-170, W. van Gool, ed. Elsevier, New York, 1973 and Takahashi et al, J. Electrochem. Soc. Vol. 124, 280-284 (1977)]. Supersonic conductors are solids with ion conductances exceeding 0.01 ohm-1 cm-1resulting from the motion of ions, not electrons [Mahan, Superionic Conductors, Mahan and Ross ed., Plenum Press, New York and London, 115, 1976]. These materials have also been called solid electrolytes or fast-ion conductors. The fluoride ion conductors have tysonite-type structure with the general formula, M.sub.x Ln.sub.y F.sub.3-x where M is an alkaline earth metal ion such as calcium, strontium or barium and Ln is a lanthanide metal ion such as lanthanum or cerium and y equals 1-x. Conductivity of these ternary fluoride compounds is reported to be about an order of magnitude greater than Pure fluorides of lanthanide metals up to about 5 mole % doping, with the best one known in the art being Ce.sub.0.95 Ca.sub.0.05 F.sub.2.95. The results in these studies supported the earlier hypothesis that mobile fluoride ions in the crystal lattice of large cationic metal fluorides are responsible for the high conductance.
Several reports have appeared recently that describe the use of single crystal fluoride electrodes for measurement of enzyme activities [Siddiqi, Clin. Chem. Vol. 28. 1962-1967 (1982) and EP 0,227,073 issued on Jan. 7, 1987]. The technique is based on the detection of H.sub.2 O.sub.2 (hydrogen peroxide) produced by various enzymatic reactions such as oxidation of glucose by glucose oxidase. The detection means comprises an interaction of H.sub.2 O.sub.2 with fluorinated aromatic compounds such as 4-fluoroaniline or 4-fluorophenol in the presence of peroxidase to generate fluoride ions which may then be measured by the fluoride ion-selective electrode. This permits rapid and simple measurements of enzyme activities and their substrates in biological fluids. These disclosures, however, do not teach the use of or how to prepare high sensitivity fluoride electrodes that are of sufficiently low cost as to be especially useful as disposable sensors for clinical applications.